I don't have the 70-200/4 but I do have the 70-300DO which I purchased because it is such an ideal travel lens. Whilst this isn't an 'L' lens per se, it is an expensive and very capable lens.
I've found the images to be sharp and I haven't noticed any of the diffusion artifacts that can occur with this technology with my pictures. (I've seen examples where there seems to be a slight 'bloom' of bright edges etc).
The 70-300 is an ideal travel package as it is (relatively) light, compact and very useable with the IS function. Although it is touted as having 2nd generation IS that works left-on on tripods my experience was that you definitely should turn this off.
In my own resolution tests I found that the 70-300 was very sharp and reasonably contrasty. I'd read Michael's advice here about applying some PK Sharpen/edge sharpen 1 to the DO images but I haven't found that necessary with my lens.
For travel, my 70-300 occupies less space than my 24-70L and feels lighter. Not being beige is another advantage for travel too.
From a cost standpoint, a 1.4x convertor with your existing lens is going to be a LOT less expensive. You'll lose the IS features and have a heavier set up but I would expect that it would still produce great results. The 70-300 will definitely be more portable and discreet.
